See Figure 1
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.
Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, the piston will be beginning the power stroke just as the compressed and ignited air/fuel mixture starts to expand. The expansion of the air/fuel mixture then forces the piston down on the power stroke and turns the crankshaft.
Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (Before Top Dead Center or BTDC). If the setting for the ignition timing is 5°BTDC, each spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the piston goes faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.
Timing marks consist of O marks or scales and can be found on the rim of the crankshaft pulley and the timing cover. The mark(s) on the pulley correspond(s) to the position of the piston in the No. 1 cylinder. A stroboscopic (dynamic) timing light hooked into the circuit of the No. 1 cylinder spark plug can be used to indicate ignition timing as follows:
Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read, since the stroboscopic flash makes the pulley appear to be standing still. Proper timing is indicated when the mark and scale are in proper alignment.
Because these vehicles utilize high voltage, electronic ignition systems, only a timing light with an inductive pickup should be used. The pickup simply clamps to the No. 1 spark plug wire, eliminating the adapter. It is not susceptible to cross-firing or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.
INSPECTION & ADJUSTMENT
See Figures 2, 3 and 4
- Start the car and let it reach operating temperature.
- Turn all accessories OFF .
- Connect a timing light according to the tool manufacturers instructions.
- Check the timing by aiming the light at the pointer on the timing belt cover. The yellow timing mark on the crankshaft pulley should line up with the pointer and should read 10 ° BTDC.
- If the timing is incorrect, loosen the distributor hold-down bolts.
- Rotate the distributor until the desired timing is achieved.
- Tighten the distributor hold-down bolts to 14-19 ft. lbs. (19-25 Nm).
- Recheck the timing and, if it is still correct, remove the light.
- If the timing is still incorrect, repeat Steps 5 through 8.